Is it mandatory to upgrade the ZCO when upgrading server

I have noticed every Zimbra Server upgrade has had a ZCO upgrade as well. I was wondering if it is mandatory to upgrade the ZCO on each client at the same time you upgrade the Zimbra NE Server software? Since I have many machines to upgrade, I want to spread out the upgrades over a period of time.

I've read on the forums that no, you don't have to upgrade ZCO when you upgrade ZCS. The general rule of thumb is that older ZCO will work with newer ZCS, but you shouldn't use a newer ZCO with an older ZCS unless advised by Zimbra to do so.